In the handling of web materials, it is common to provide a tubular element or sleeve to constitute a core of the machine which winds or unwinds a coil of the web material.
The web material, e.g. a paper web derived from a paper-making machine, can have its free end adhered to the core by an adhesive or glue.
The core can be engaged, in such machines, on an expanding mandrel which can comprise a pin about which two segmental gripping elements are disposed for radial displacement in a cage, bracing elements being provided between each gripping element and the pin.
The outer surfaces of the segments, therefore, engage the inner surface of the tubular member or core, while inner surfaces of these segments are urged outwardly by the bracing elements.
The pin and the bracing elements are so arranged vis-a-vis one another so that a relative displacement of the segments and the pin can transform a torque into a radial displacement of the segments to press them against the inner wall or surface of the core and retain the core on the thus expanded mandrel. Reverse rotation of the pin enables retraction of the segments and release of the core.
In the past, the outer surfaces of the gripper segments were either largely smooth or provided with a zigzag serrated configuration with sharp crests. A smooth surface has the advantage that the cardboard core is not damaged at the inner surface regions engaged by the segments of the expanding mandrel and are not abraded. However, there is of course the disadvantage that the torque transfer from the mandrel to the core is limited and less reliable.
Toothed or serrated segments as are described in German patent document DE-OS No. 33 29 330 (see U.S. Pat. No. 4,635,872) have, of course, the advantage that the crests of the serrations can bite into the relatively softer inner surface of the cardboard core and, in a form-locking manner insure positive torque transfer or development of the necessary moments.
This system, while generally highly effective, has been found to have the disadvantage that the braking moments applied to the sleeve or core can result in a significant abrasion or tearing of the inner surface of the core into which the teeth bite.
The distortion of the surface of the core where it engages the teeth and the nature of the toothed engagement of the segments in the cardboard of the core can be so extreme that release of the core after the coil has been unrolled from it is difficult.
The abrasion of the inner surface of the core with pointed or edged teeth can also cause significant local stresses in the core to the point that the core structure itself is damaged, especially with torque overloads as may occur from time to time in the unrolling operation.
Because the local stresses are so significant, the intermittent application of substantial torques and their sudden terminations can contribute to deterioration, not only of the inner surface of the core, but also of the basic core structure as a whole.
In earlier devices, it has been the practice to provide the rotary pin as a collateral polygon in cross section, having grooves in which the bracing elements, which can have a complementary cross section to the groove, can engage. The bracing elements can also have ball-shaped or flat contact surfaces in engagement with the segments.
These systems are complex and time consuming to fabricate, and, of course, are comparatively inexpensive. This is especially the case because of the need of the aforementioned grooves in earlier expanding mandrel systems and the requirement that the bracing elements have convex contours conforming to the contours of the groove.